This invention relates to the use of nuclear explosives in the production of natural gas from subterranean formations. More particularly, this invention relates to a system for reducing the amount of radio-activity released to the biosphere in the course of producing natural gas from a reservoir stimulated by the detonation of one or more nuclear explosives in gas bearing strata.
The growing shortage of natural gas reservoirs from which gaseous hydrocarbons can be produced by conventional means has led to a growing interest in the possibility of recovering natural gas from so-called tight or impermeable formations after fracturing the formations by detonating nuclear explosives therein. Two natural gas stimulation projects, i.e., Project Gasbuggy and Project Rulison, have demonstrated the technical feasibility of using nuclear explosive to stimulate the production of natural gas from such formations. However, concern has been expressed by some regarding the radioactivity that is produced in natural gas reservoirs by the detonation of nuclear explosives therein and the release of that radioactivity to the biosphere in connection with the production of the gas therefrom.
The testing programs utilized in connection with projects Gasbuggy and Rulison each included the flaring (burning) of large quantities of gas. It can be anticipated that future nuclear gas stimulation projects, whether developmental or production, will also involve the flaring of fairly large quantities of gas. Of course flaring gas from a new reservoir in order to calibrate the flow prior to placing the output on a distribution line is a common practice. In the case of a nuclearly stimulated reservoir, however, the first gas produced therefrom comes from the nuclear chimney and may contain as much as 40% CO.sub.2. Also, the radioactivity of the gas - due primarily to tritium (.sup.3 H) and .sup.85 krypton -- is highest in the first gas produced but decreases at an accelerating rate.
Accordingly, the most effective approach to the development of a particular nuclearly stimulated natural gas reservoir to the point where the gas produced therefrom is of sufficiently high BTU content and sufficiently low radioactivity to be commercially acceptable may be to flare some volume of gas representing some fraction of each chimney volume. The fraction of a chimney volume flared would vary from gas field to gas field depending, among other things, upon the quantity, BTU content and radioactivity of the gas with which the gas from the nuclear stimulated reservoir would be diluted prior to delivery to the consummer.
As indicated in Report No. PNE-R-57, "Project Rulison, Final Operational Report, Production Tests," available from the National Information Service, U.S. Department of Commerce, the total radioactivity estimated to have been released to the environment during the Project Rulison production tests through flaring a total of 455 M.sup.2 CF (million standard cubic feet) of gas was 2824 curies of .sup.3 H, 1,064 curies of .sup.85 Kr, 2.4 curies of .sup.14 C, 0.00011 curies of .sup.203 Hg and a few curies of .sup.37,39 Ar and naturally occurring .sup.222 Rn. The radioactivity concentrations in the gas ranged from 185 pCi/cc of .sup.3 H, 145 pCi/cc of .sup.85 Kr and 0.35 pCi/cc of .sup.14 C after a calibration flaring of a total of 12.5 M.sup.2 CF to about 3.3 pCi/cc of .sup.3 H, 2.8 pCi/cc of .sup.85 Kr and 0.07 pCi/cc of .sup.14 C at the end of the flaring of a total of 455 M.sup.2 CF.
The dispersal of the radioactivity effected by the Rulison flare stack resulted in an actual population dose of a very low level. The Environmental Protection Agency estimates that the population dose resulting from the Project Rulison flaring of the 455 M.sup.2 CF alluded to above was on the order of about 0.001 mrem. However, there is an incentive, particularly from a public relations standpoint, to reduce the contribution to the population dose attributable to nuclear stimulation projects to as low a value as practicable.